Mechanical gearboxes are extensively used to match the operating speed of prime-movers to the requirements of their loads for both increasing rotational speed such as, for example, in a wind-powered generator or reducing rotational speed such as, for example, in an electric-ship propulsion arrangement. It is usually more cost and weight effective to employ a high-speed electrical machine in conjunction with a mechanical gearbox to achieve requisite speed and torque characteristics. However, while such a high-speed electrical machine in conjunction with a mechanical gearbox allows high system torque densities to be realised, such mechanical gearboxes usually require lubrication and cooling. Furthermore, reliability can also be a significant issue. Consequently, direct drive electrical machines are employed in applications where a mechanical gearbox cannot be used.
Several techniques of achieving magnetic gearing, using permanent magnets, are known within the art. For example, FIG. 1 shows the most commonly used topology for magnetic gears. It can be appreciated that FIG. 1 shows a magnetic gear 100 comprising a first, high-speed, rotor 102 bearing a plurality of permanent magnets 104 that is magnetically coupled, in a geared manner, to a second, low speed, rotor 106 comprising a number of permanent magnets 108. A significant disadvantage of the magnetic gear 100 shown in FIG. 1 is that the topology suffers from a very poor utilisation of the permanent magnets since very few of the permanent magnets simultaneously contribute to torque transmission at any given time. The very poor torque transmission capability has limited the use of magnetic gearing.
The problem associated with the magnetic gear 100 of FIG. 1 is solved by the magnetic gear 200 shown in FIG. 2. FIG. 2 shows a rotary magnetic gear 200 comprising a first or inner rotor 202, a second or outer rotor 204 and a number of pole pieces 206, otherwise known as an interference or an 5 interference means. The first rotor 202 comprises a support 208 bearing a respective first number of permanent magnets 210. In the illustrated magnetic gear, the first rotor 202 comprises 8 permanent magnets or 4 pole-pairs arranged to produce a spatially varying magnetic field. The second rotor 204 comprises a support 212 bearing a respective second 10 number of permanent magnets 214. The second rotor 204 comprises 46 permanent magnets or 23 pole-pairs arranged to produce a spatially varying field. The first and second numbers of permanent magnets are different. Accordingly, there will be little or no useful direct magnetic coupling or interaction between the permanents magnets 210 and 214 such that rotation of one rotor will not cause rotation of the other rotor.
Pole pieces 206 are used to allow the fields of the permanent magnets 210 and 214 to interact in a geared manner. The pole pieces 206 modulate the magnetic fields of the permanent magnets 210 and 214 so they interact to the extent that rotation of one rotor will induce rotation of the other rotor in a geared manner. Rotation of the first rotor 202 at a speed ω1 will induce rotation of the second rotor 204 at a speed ω2where ω≠ω2. The gear ratio is directly related to the ratio of the number of pole-pairs on the outer rotor to the number of pole-pairs on the inner rotor. In the given example, which has 23 pole-pairs on the outer rotor and 4 pole-pairs on the inner rotor, the 25 gear ratio is 5.75:1. Therefore, in a magnetic gear, the rotors always have a different number of pole pairs.
However, the magnetic gear topology shown in FIG. 2 has the disadvantages that it is large in size for given operating conditions. That is, the magnetic gear must be designed to transmit a peak torque level to be encountered in operation, even though that peak torque may not be encountered more than momentarily. Hence the magnetic gear is large in size and expensive to produce. Further, iron losses are significant, for the magnetic flux which is provided by the permanent magnets must be sufficient to couple the rotors when subjected to the peak torque, even though this peak torque rarely occurs.
It is an object of embodiments of the present invention to at least mitigate one or more of the above problems of the prior art.